TRIPOLI, April 14 (Reuters) - Saadi Gaddafi and Saif al-Islam, two of deposed Libyan leader Muammar Gaddafi's sons, are expected to appear in court on Monday, facing charges of corruption and war crimes alongside more than 30 other Gaddafi-era officials.

The mass trial will be seen by the international community as a barometer for Libya's progress in establishing a democratic state after the chaotic 2011 revolution that ended four decades of Gaddafi's one-man rule.

Post-Gaddafi Libya has so far been defined by a weak interim government and growing unrest as former revolutionary fighters refuse to give up their weapons, and armed protesters blockade the country's crucial oil exports.

The North African state's nascent democracy is struggling to establish basic institutions and rule of law as Gaddafi left behind only a husk of a government after absorbing all the power into his own hands.

The trial will begin a day after interim prime minister Abdullah al-Thinni announced his resignation after an attack on his family and following the ousting of former prime minister barely one month ago.

"If they don't get fair trials then it casts doubt over whether the new Libya is not about selective justice," Hanan Salah, Libya researcher in the Middle East and North Africa division at Human Rights Watch said.

"So far, there have been problems with legal representation. Many of those on trial did not have a lawyer from the beginning - a cornerstone of a fair trial."

The International Criminal Court and other human rights organisations are concerned over the fairness of Libya's justice system although the government won the right last year to try Gaddafi's former spy chief domestically instead of at the ICC.

Saadi Gaddafi, known as a playboy with a brief career in professional soccer, was extradited to Libya from Niger in early March. He may appear in a Tripoli court for the first time to hear charges. But that may depend on whether investigators have finished gathering evidence.

The head of the government investigations could not confirm details about the trial.

"I think Saadi will not appear in court tomorrow, as investigations are still on-going," Libya's state prosecutor Abdelqadir Radwan told Reuters by phone late on Sunday.

Gaddafi's more prominent son, Saif al-Islam, long viewed as his heir, is expected to appear by video-link inside the courtroom. He is being held by the powerful western Zintan militia group, who have refused to hand him over to the central government they believe cannot provide a secure trial.

Gaddafi's ex-spy chief Abdullah al-Senussi will also appear in court on Monday along with the former foreign minister Abdul Ati al-Obeidi.

"We have had many cases where the defendants' lawyers were not allowed to review evidence and get access to court documents in the pre-trial phase (the pre-trial chamber)..." Salah said.

"In some other unrelated cases, judges and lawyers were harassed and there are allegations of forced confessions."

Libya's justice minister insisted that the trial was open the public who would ensure the process was fair and not turned into a "Mickey Mouse" show trial.

"I will not allow any crazy stuff, I will make sure it meets international standards ... that is why we are having open trials," Salah al-Merghani, the justice minister told Reuters.

"We heard there were complaints from the lawyers... The court will see if the complaints are genuine or not." (Editing by Eric Walsh)